feat(prompt): make context-file truncation limit configurable

PROBLEM: Automatic context files such as SOUL.md and AGENTS.md were capped by a hardcoded CONTEXT_FILE_MAX_CHARS value. Amy's local fork had raised that constant from 20K to 25K so a larger SOUL.md would not be silently truncated, but the hardcoded 25K value changed upstream default behavior and made the patch less generally useful.

SOLUTION: Restore the upstream-compatible 20K default, add a context_file_max_chars config setting for users who intentionally keep larger identity/project-context files, keep chat-visible truncation warnings, and document the new setting. Tests cover the default, config override, explicit max_chars precedence, and the warning text.

website/docs/developer-guide/prompt-assembly.md

@@ -128,7 +128,7 @@ def load_soul_md() -> Optional[str]:
         return None
     content = soul_path.read_text(encoding="utf-8").strip()
     content = _scan_context_content(content, "SOUL.md")  # Security scan
-    content = _truncate_content(content, "SOUL.md")       # Cap at 20k chars
+    content = _truncate_content(content, "SOUL.md")       # Cap defaults to 20k chars, configurable
     return content
 ```
 
@@ -195,7 +195,7 @@ def build_context_files_prompt(cwd=None, skip_soul=False):
 
 All context files are:
 - **Security scanned** — checked for prompt injection patterns (invisible unicode, "ignore previous instructions", credential exfiltration attempts)
-- **Truncated** — capped at 20,000 characters using 70/20 head/tail ratio with a truncation marker
+- **Truncated** — capped at `context_file_max_chars` characters (default 20,000) using 70/20 head/tail ratio with a truncation marker
 - **YAML frontmatter stripped** — `.hermes.md` frontmatter is removed (reserved for future config overrides)
 
 ## API-call-time-only layers